This invention relates to a process for the production of nitrogen by low-temperature rectification of air, in which the air feed stream is compressed and divided into two partial air streams, the first of which is cooled and fed to a rectification, while the second is separately further compressed, cooled, expanded and also fed to the rectification wherein a nitrogen product stream is discharged from an upper section of the rectification. The invention further relates to an apparatus for performing such a process.
A process of the type mentioned above is disclosed in DE-OS 30 35 844. This reference shows a process for obtaining oxygen of average purity, in which--especially in the embodiment according to FIG. 2--a nitrogen stream is obtained at basically atmospheric pressure. In this process the first partial air stream, after its cooling, is fed directly into a rectifying column. The second, further compressed, partial air stream, after its cooling, is brought into heat exchange with separation products still in heat exchange with the bottom liquid from the rectification and then throttle expanded, before it is also fed into the rectifying column. A nitrogen product stream is taken from the head of the rectifying column and expanded to about atmospheric pressure while producing work. Most of the work obtained by expansion of the nitrogen stream is transferred to the compressor for separately compressing the second partial air stream.
This process exhibits the drawback that the nitrogen, obtained from the rectification at a pressure of about 3.3 bars, must be expanded to about atmospheric pressure in order to drive the second partial air stream compressor. If nitrogen at a higher pressure is desired by the consumer, the nitrogen must be compressed from atmospheric pressure to the desired pressure, thus increasing costs. If the nitrogen obtained at the elevated pressure from the rectification is delivered to the consumer without previous expansion, additional energy must be expended for compression of the second partial air stream, which is also uneconomical.